Aircraft engine pressure ratio autotrim system

ABSTRACT

An automatic thrust management system for gas turbine engines in single or multi-engine aircraft is disclosed. An engine pressure ratio limit is automatically calculated using known techniques and compared with the actual engine pressure ratio of the engine having the maximum pressure ratio for the purpose of an automatic control system. In one mode of operation, the throttles for each engine are trimmed to maintain the engine pressure ratio at the limit value. In another mode of operation, each engine is trimmed as necessary to insure that the engine pressure ratio does not exceed the limit value. The automatic thrust management system of this invention may be incorporated together with existing autothrottle systems in aircraft which control the engine throttles as a function of airspeed error.

I United States Patent 1191 [111 3,852,956 Martin Dec. 10, 1974 [54]AIRCRAFT ENGINE PRESSURE RATIO 3,299,701 1/1967 Scarlett 73/178 RAUTOTRIM SYSTEM 3,697,731 10/1972 Kempema et al...... 235/1502 3,736,7966/1973 Hohenberg 73/178 T [75] Inventor: Anthony N. Martin, Simsbury,

Conn Primary ExaminerTrygve M. Blix [73] Assignee: United AircraftCorporation, East Assistant Examiner-Stephen Kunin H tf d C Attorney,Agent, or Firm-Donald F. Bradley [22] Filed: Dec. 3, 1973 ABSTRACT [21]Appl 4 An automatic thrust management system for gas tur- RelatedApplication Data bine engines in single or multi-engine aircraft is dis-[62] Division of Ser NO 274 123 July 21 1972 Pat No closed. An enginepressure ratio limit is automatically 3,813,063 calculated using knowntechniques and compared with the actual engine pressure ratio of theengine having 52 us. c1 60/39.1s, 60/3928 R, 60/224, the maximum WSWreratio for the Purpose of 73/1173, 235/150 21 tomatic control system. Inone mode of operation, the 511 Int. Cl F020 7/02, F02g 1/06 throttlesfor each engine are trimmed to maintain the [58] Field of Search60/3915, 3916, 3928 R, engine pressure ratio at the limit value. Inanother 60/224; 73/1173, 1174 178 R 178 T; mode of operation, eachengine is trimmed as neces- 235/150'21, 1502; 244/77 R 77 D 77 F; saryto insure that the engine pressure ratio does not 340/27 R 27 NA exceedthe limit value. The automatic thrust management system of thisinvention may be incorporated to- [56] References Cited gether withexisting autothrottle systems in aircraft UNITED STATES PATENTS whichcontrol the engine throttles as a function of airspeed error. 3,l74,2843/1965 McCarthy 235/1502] X 3,238,768 3/l966 Richardson 235/1502 x 4Claims, 3 Drawing Flgures fl ea UPT/Q/M /'/V6'//Vf 52 /A////5/7 UP7/( /M6P6 Mfl/ Z7 PATENIEU DEC 10 um 3,852,956 SHE-2E7 10F 3 PAIENTEHBEc 10197 4 1 SHEEI 2 BF 3' xiwk AIRCRAFT ENGINE PRESSURE RATIO AUTOTRIMSYSTEM This is a division, of application 'Ser. No. 274,123, filed July21, 1972, now US. Pat. No. 3,813,063.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to gas turbine engines for aircraft, and in particular to anautomatic thrust management system for multi-engine aircraft. Theautomatic thrust management system provides automatic regulation of thethrottles or power levers for each engine to insure that the enginethrust ratio does not exceed a value which will cause damage to oroverly reduce the service life of the engine. In one mode of operationof the invention, the engine throttles are adjusted so that each enginewill be operating in its most efficient manner at or near the enginepressure ratio limit. The automatic thrust management system may be usedconcurrently with existing autopilot or autothrottle systems in aircraftto prevent duplication of engine throttle controls thereby reducing thecost of installation. Existing autothrottle systems can only be usedduring a small fraction of the flight time, often the descent phaseonly. The systemdescribed herein permits automatic operation of thethrottles throughout all phases of the flight.

In another embodiment of the present invention, the fuel control foreach engine is automatically trimmed so that each engine is operating atthe same pressure ratio to thereby prevent thrust unbalance between theengines due to errors in throttle settings or small changes in engineperformance characteristics.

2. Description of the Prior Art It is frequently necessary to fly anaircraft with the engines operating at the maximum thrust of which theyare capable, without at the same time causing damage or severedeterioration to the engines due to exceeding predefined settings. Thisthrust setting is a function of the engine pressure ratio limit whichcan be permitted, and this ratio is dependent upon the actual flightmode (take-off, climb, cruise, etc.) and also varies continuously as afunction of altitude, Mach number, total air temperature and the amountof bleed air extracted from the engine for anti-icing systems etc,Currently this engine pressure ratio limit (EPRL) is calculated by theflight crew during critical portions of the flight, typically climb, oruse is made of an electronic computer which establishes the EPRLcontinuously and displays it on an indicator. The crew has the task ofattempting to control to this limit during climb by continuouslymodifying the position of the power levers or throttles for each engineas the EPRL values change. This imposes a severe work load on the flightcrew during a critical portion of the flight, and in practice they arenot able to perform this task to the standard required. This requirementtypically is to adjust the actual engine pressure ratio (EPR) to withinplus or minus 0.01 of the EPRL throughout the climb phase. consequently,the engines are being deteriorated or damaged due to exceeding this EPRLresulting in a lower service life, or engine operation is inefficientresulting from the actual EPR being held far below the EPRL by othermore cautious flight crews. EPR excursions below the maximum ratingshave also contributed to the inability to obtain economical cruisealtitude prior to leaving radar air traffic control area in someinstances.

An additional problem with multi-engine aircraft is that the mostefficient operation of the aircraft, particularly during cruise and alsoduring the final landing phase, is only achieved if the engine powersare balanced one against the others. This necessitates continued smalladjustments of throttlepositions, to which task the flight crew is notalways able to give the required attention. Also there is a need tofrequently manually trim individual engine control systems as a groundbased operation to maintain engine thrust characteristics as the enginesdeteriorate during service life, or whenever componenet parts of thecontrol or linkage system are changed.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided an automatic aircraft engine pressure ratio control systemor automatic thrust management system in which the engine pressure ratiois automatically maintained at the engine pressure ratio limit tothereby provide maximum thrust during climb or other critical maneuversof an aircraft. This mode of operation, referred to as the EPR controlmode, automatically computes the engine pressure ratio limit (EPRL) andcompares it with the maximum EPR at which any of the engines isoperating. If there is a difference between the EPRL and the maximumEPR, an error signal is fed to the existing autothrottle system in theaircraft to cause the throttles for all engines to be movedsimultaneously in a direction that will eliminate the error signal andmaintain the engines at or near the EPRL. If an autothrottle system isnot present in the aircraft, additional means must be provided to movethe throttles of all engines in response to the error signal. Such meanscould either operate on all throttles simultaneously with a singleoutput signal or on each throttle individually where suitable drivemechanisms are provided.

In accordance with another aspect of the present invention, an EPRlimiting mode operation may be provided such as during aircraft cruisewhen maximum thrust is not required. In this mode of operation, theengines are regulated by the existing autothrottle system and maintainthe desired cruise speed as long as the engine EPRs are below the EPRL.Whenever the engine EPR exceeds the EPRL, all throttles are reducedautomatically. v

In accordance with another aspect of the present invention, there isconnected into the autothrust management system an automatic modecontrol which will select the proper control or limiting mode dependingon the desired flight condition. If the automatic mode is selected, thepilot must inform the automatic mode selector whether the aircraft is tooperate in a climb, cruise, take-off or any other flight condition.

In accordance with another embodiment of the present invention, anengine autotrim or pressure ratio equalization control is provided whichwill automatically maintain or adjust each engine control or linkagesuch that each engine EPR is equal to the highest engine EPR which mayor may not be at the limit. The actual EPR of each engine is comparedwith the maximum EPR of all engines, and a trim motor connected with theengine fuel control is actuated to adjust upward the EPR of that engineto equal that of the maximum EPR. A zero trim reference switch isprovided in each fuel control trim motor, and all trim motors areautomatically downtrimmed whenever none of the trims are at a minimum inorder to maintain the authority of the fuel control trim motors and toprevent the trim motors from reaching a physical stop due to possibleupward escalation of the trim motor positions following successivemanual changes of throttle positions during aircraft operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram ofthe basic autothrust management system control.

FIG. 2 is a schematic block diagram of the autotrim system.

FIG. 3 is a schematic block diagram showing how the basic functions ofthe systems of FIGS. 1 and 2 may be performed by a multi-purposecomputer in conjunction with an autothrottle system and a fuel controltrim motor for each of the aircraft engines.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring particularly to FIG.1, there is shown in block diagram form the basic automatic thrustmanagement system which operates in conjunction with the existingautothrottle system in multi-engine aircraft. An

7 autothrottle system traditionally drives the throttles of all enginesas a function of airspeed error, computed by comparing aircraft airspeedwith a desired airspeed selected by the pilot, and fed as an input intothe autothrottle system. The described automatic thrust managementsystem takes advantage of the airspeed error (IASJ signal and thethrottle drive motor already present in aircraft equipped with anautothrottle system. The additional functions required by the autothrustmanagement system of this invention may be performed by conventionalanalog type electronic circuitry, or by means ofa multi-purpose digitalcomputer as shown in conjunction with FIG. 3.

As indicated previously, the engine pressure ratio limit (EPRL) isnormally calculated by the flight crew, although some aircraft nowmake'use of a computer which automatically establishes the EPRLcontinuously and displays it on an indicator. In either case, the flightcrew must continually adjust the throttles, particularly during climb,to assure that the EPRL is not exceeded, and on the other hand to insurethat the engines are producing maximum thrust by running as close aspossible to the EPRL. If an EPRL computer is already installed in theaircraft, its output may be utilized as an input to the presentautothrust management system. In no EPRL computer is available, the EPRLmust be computed within the autothrust management system.

Shown in FIG. 1 is an EPRL computer 10 to which is supplied inputsindicative of aircraft altitude (ALT), Mach number (M,,), total airtemperature (TAT) and engine bleed air, The computation of the EPRL fromthe inputs according to the specific engine performance curves is wellknown and may be performed by many existing devices and will not bedescribed in detail.

The output from the EPRL computer, a signal indicative of the EPRL forthe particular aircraft flight conditions, is fed through a signal line12 to a comparator 14. Also fed to comparator 14 is a signal indicativeof the maximum EPR of the engines in the aircraft. Assuming a standardfour engine passenger aircraft, the actual engine pressure ratio (EPR)from each of the four engines sensed by means of transducers at theappropriate engine stations, is fed to a maximum selector 16 from whichthe EPR signal of maximum value is selected and fed through line 18 tothe comparator 14. The maximum EPR signal is also fed through line 27 toFIG. 2 as will be described subsequently. Comparator l4 compares theEPRL signal with the maximum EPR signal and produces an EPR error signal(EPR which is fed on signal line 20 through an EPR compensation network22 and into a mode selector 24. The EPR signal is also fed through asignal line 26 to an automatic mode logic block 28 whose output isconnected through a signal line 30 and a switch 32 to the mode selectorblock 24.

As previously described, the existing autopilot system compares thedesired pilot selected airspeed with the actual aircraft airspeed, andgenerates an indicated airspeed error (IAS This signal is utilized inthe present automatic thrust management system. The IAS signal is fedthrough a signal line 34 into an IAS compensation network 36 and theninto the mode selector block 24. The IAS signal is also fed through asignal line 38 to the automatic mode logic block 28.

The mode selector block 24, which may be conventional electroniccircuitry or a portion of a multipurpose computer, consists of a switchwhich connects either the EPR; signal or the IAS, signal to theautothrottle system 44. The switch is shown schematically in block 24,but may be electronic logic circuitry or a computer. When the switch isset to pass the EPR,. input the system operates in an EPR control modeand the power levers or throttles shown schematically at 46 will bemoved simultaneously by the signal from autothrottle system 44 toincrease or decrease the engine setting to increase or decrease powerand thereby to null the EPR signal. The pilot can enter this EPR controlmode either by manual selection using switch 40, or automatically byusing switch 32 and selecting a climb flight mode with switch 52. Thismode will normally be selected during climb, or during other aircraftflight conditions involving heavy loads, that is, in genera] whenmaximum thrust is desired.

The alternative mode of operation is an EPR limiting mode which willnormally be used during cruise or descent of the aircraft. It causes theaircraft to operate at the air-speed selected by the pilot, unless thisrequires such a high power that the EPR exceedsv the EPRL. In this eventthe control will maintain EPR equal to EPRL until such time as theairspeed reaches that selected by the pilot. This may occur eitherbecause the pilot selects a lower desired air-speed or more typicallyduring cruise because the aircraft weight is decreasing as fuel isconsumed and the aircraft is consequently accelerating while maintainingEPR equal to EPRL and a constant altitude. The pilot can enter this EPRlimiting mode by closure of switch 32 and selection of the cruise flightmode. This will cause the automatic mode logic 28 to set the modeselector 24 so as to connect the IAS signal to the autothrottle system44. The power levers or throttles shown schematically at 46 will then bemoved simultaneously by the lAS signal to increase or decrease theengine setting to increase or decrease power and thereby to null the IASsignal. In the event, however, that the EPR signal 26 received by theautomatic mode logic 28 decreases to zero, thereby indicating that EPRhas reached EPRL, the mode selector 24 will be operated so as to connectthe EPR signal to the autothrottle system 44. The automatic mode logic28 will maintain the mode selector 24 in this position until it detectsthat lAS signal 38 is equal to zero, indicating that the desiredairspeed has been achieved. It will then restore that IAS signal and theairspeed will be maintained at that selected by the pilot unless theEPRL is again exceeded due to further changes in air- I conditiondesired and the relationship between the signals, with limits placedthereon in various modes. In addition, the control performance may beimproved for specific aircraft applications by incorporating signaldeadbands, time delays or other traditional dynamic compensationfunctions into the automatic mode logic In the present embodiment, withthe only inputs to the automatic mode logic block 28 being climb andcruise,a simplified logic may be used. The logic may be simpleelectronic switching circuitry, or the logic may be performed by aspecial purpose or multiple purpose computer. This would facilitate manyvariations on the control logic. For example, when the pilot selects acruise mode of flight by closing switch 54 and when the IAS signal isvery much smaller than the IAS selected by the pilot, resulting in alarge IAS signal, this condition of operation is indicative of anaccelerating flight condition and the mode selector block 24 coulddirectly select the EPR,. signal as the controlling signal. However,where the IAS signal is relatively close to the IAS selected by thepilot, resulting in a small IAS, signal, the mode selector 24 wouldselect the IAS to be controlling. In this latter case, where the IAS,signal controls throttle position, the EPR signal must always bepositive indicative of the fact that the EPR has not exceeded the EPRL.Thus, as described, during the climb flight mode, the engines willalways be EPR limited. During cruise, the engines may either be EPRlimited (during acceleration or heavy load) or will follow the standardautopilot IAS input, but even in the latter case, the engine pressureratio cannot exceed the engine pressure ratio limit.

FIG. 2 shows the engine autotrim or en'gine pressure ratio equalizationsystem which may be used in conjunction with the autothrust managementsystem of FIG. 1, but which also may be used independently. The purposeof the engine autotrim system is to maintain each of the engines at thesame engine pressure ratio in order to balance the thrust produced byeach engine. This would relieve the pilot of the. necessity to modifyeach throttle lever individually by small amounts to balance the EPRs ofeach engine. To achieve this result, a trim screw is provided in thefuel control of each engine, the trim having limited authority to adjustthe fuel flow so that differences in the pressure ratios of each enginemay be eliminated.

FIG. 2 shows the components of the engine autotrim system. One of theautotrim systems is associated with each engine. A maximum EPR signalproduced by comparing the EPRs from each engine and selecting themaximum signal therefrom is produced on signal line 27 as shown inFIG. 1. The maximum EPR signal is fed to a comparator 60 where it iscompared with the actual EPR signal from the selected engine fed to thecomparator on signal line 62. For that engine which produces the maximumEPR signal, the comparator 60 will produce a zero error signal; for allother engines, the actual EPR on signal line 62 will be less than themaximum EPR signal on line 27, and an EPR signal will be generated bycomparator 60 on line 64. The EPR signal is fed to an uptrim inhibitblock 66, the purpose of which will be explained later, and is then fedto uptrim control 68 which in turn feeds a signal to trim motor drivelogic block 70. The trim motor drive logic produces the necessary drivepulses which are then fed through line 72 to an engine trim motor 74,generally a stepper motor. The engine trim motor 74 is connected to atrim screw in the engine fuel control downstream from the main throttleinput to the control. The trim motor will have a limited authority,generally in the range of 20 percent of the fuel flow. In response tothe actuation of the engine trim motor, the fuel flow will be increasedso that the actual EPR in the selected engine will be equivalent to theEPR maximum whereby the EPR signal is nulled.

Because the engine autotrim system of FIG. 2 will only allow uptrims ofthe engine trim motor 74, and because there is a limited trim range withphysical stops, means are provided to prevent the engine trim motor 74on the engine autotrim systems from becoming blocked at the upper limit.There is provided in connection with the engine trim motor 74 a limitswitch 76 which will automatically close when the engine trim motor 74is at its lower limit. A signal is fed from the switch 76 to an ANDcircuit 78. Only one AND circuit 78 is required for each aircraft. Aslofed as inputs to AND circuit 78 are identical signals from the limitswitches connected with the other engine trim motors. If none of thelimit switches 76 are closed, thereby indicating that none of the enginetrim motors are at the lower limit, AND circuit 78 will produce anoutput signal which is fed through a signal line 80 to a downtrimnetwork 82 and to the uptrim inhibit 66 in each of the engine autotrimsystems. Downtrim circuit 82 will feed a signal which instructs the trimmotor drive logic to drive the engine trim motor 74 in a direction whichwill reduce the fuel flow to each of the engines while at the same timethe uptrim inhibit 66 will prevent a conflicting uptrim signal beingsupplied. Thus, all engines are downtrimmed simultaneously. When the en-.gines are downtrimmed a sufficient amount that the limit switch 76 onat least one of the engine trim motors is closed, the output from theAND circuit 78 will reverse and no further downtrim is provided.

By downtrimming all of the engines, the engine pressure ratios in eachof the engines will be reduced an equal amount, and there will therebybe no change in the EPR signal on line 64. However, there will be achange in the EPR,. signal of FIG. '1, and if the aircraft is operatingin one of the modes in which the EPR, signal of FIG. 1 is controlling, asignal will be fed to the autothrottle control 44 and all throttles 46will be moved in a direction to increase all engine fuel controlsettings and thereby all engine pressure ratios. The performance of thesystem for specific aircraft applications could be optimized by thevarious available control dynamic compensation functions beingincorporated into the trim motor drive logic.

It is also apparent that where the automatic thrust management system ofFIG. 1 is not provided, the reference signal EPR from FIG. 1 could bedenied within the engine autotrim system which would in that situationbe connected to the other engine autotrim systems instead of to theautomatic thrust management system. In this event an average EPR couldalternatively be used as the reference in place of the EPR with somebenefit inperformance for specific aircraft applications.

The embodiment of FIG. 3 shows how a multipurpose computer may be usedto implement the functions of FIGS. 1 and 2. The heart of the system isan automatic thrust management computer 90. Inputs to the computer 90include those from the mode selector block 92, equivalent to the pilotactuated mode switches shown in conjunction with FIG. 1, and inputs froma central air data computer 94 which include altitudes, Mach numberand'total air temperature. These signals could provided by individualsensor inputs to computer 90 in aircraft systems where a central airdata computer is not already available. Also fed as an input to computer90 is the indicated airspeed error as determined by the autopilot andshown as IAS block 96.

Inputs to the computer 90 are also received from each engine 98. Theengine pressure ratios, or the parameters necessary for determining theengine pressure ratios in computer 90, are fed from engine parametersensors 100 connected with each engine 98.

The automatic thrust management computer 90 is programmed to perform thefunctions shown in FIGS. 1 and 2. An output signal is fed from thecomputer 90 to the. autothrottle system 102 which controls throttles104, these blocks being equivalent to corresponding blocks 44 and 46 ofFIG. 1. Outputs are also fed from computer 90 to the fuel control trimmotors 108 connected with each engine 98, the trim motor 108 beingequivalent to trim motor 74 of FIG. 2. The functions of both FIGS. 1 and2, or'either, may be performed by the computer 90 depending upon thetype ofcontrol desired in a particular aircraft.

The present invention thereby provides automatic control of thethrottles of each engine during critical portions of an aircraft flight,and enhances engine life by preventing engine pressure ratio fromexceeding a specified limit. The system also improves aircraft operatingefficiency thereby insuring that engines are oper ating at the maximumlimit when necessary. The present invention also minimizes the need forground trimming of engine fuel controls and insures that all engines areoperating at equivalent thrust levels.

Although the present invention has been described in terms of itspreferred embodiments, it is apparent to those skilled in the art thatchanges and modifications thereof may be made without departing from thescope of the invention as hereinafter claimed.

I claim:

1. In an aircarft having a plurality of turbine engines, an engineautotrim system for maintaining each of said plurality of engines at thesame pressure ratio comprising means for determining the pressure ratioacross each of said plurality of engines and producing an enginepressure ratio signal indicative thereof,

means for selecting from said plurality of engine pressure ratio signalsthe maximum of said signals,

means for comparing each of said plurality of engine pressure ratiosignals with said maximum signal to produce for each engine a pressureratio error signal proportional to the difference therebetween,

a fuel control for each of said plurality of engines for varying theflow of fuel thereto,

fuel trimming means connected with each of said fuel controls andadapted to vary the fuel flow to each said respective engine, said fueltrimming means having a limited fuel adjustment range,

and means for connecting each said pressure ratio error signal with thefuel trimming means for its respective engine to actuate said fueltrimming means and vary the flow of fuel to the engine in an amount anddirection to null said pressure ratio error signal.

2. An engine autotrim system as in claim 1 in which said fuel trimmingmeans is a trim screw located in a fuel flow passage to said enginedownstream from said fuel control.

3. An engine autotrim system as in claim 1 further comprising a limitswitch connected with each of said fuel trimming means, said limitswitch being closed and producing a limit signal only when therespective fuel trimming means is at the limit ofits fuel adjustmentrange whereby the minimum fuel flow passes therethrough,

and means responsive to the absence of a limit signal from any of saidlimit switches to actuate the fuel trimming means for all of saidengines in a direction to reduce the flow of fuel therethrough.

4. An engine autotrim system as in claim 3 further comprising means fordisconnecting the pressure ratio error signal from all of said fueltrimming means when all of said limit switches are open.

1. In an aircarft having a plurality of turbine engines, an engineautotrim system for maintaining each of said plurality of engines at thesame pressure ratio comprising means for determining the pressure ratioacross each of said plurality of engines and producing an enginepressure ratio signal indicative thereof, means for selecting from saidplurality of engine pressure ratio signals the maximum of said signals,means for comparing each of said plurality of engine pressure ratiosignals with said maximum signal to produce for each engine a pressureratio error signal proportional to the difference therebetween, a fuelcontrol for each of said plurality of engines for varying the flow offuel thereto, fuel trimming means connected with each of said fuelcontrols and adapted to vary the fuel flow to each said respectiveengine, said fuel trimming means having a limited fuel adjustment range,and means for connecting each said pressure ratio error signal with thefuel trimming means for its respective engine to actuate said fueltrimming means and vary the flow of fuel to the engine in an amount anddirection to null said pressure ratio error signal.
 2. An engineautotrim system as in claim 1 in which said fuel trimming means is atrim screw located in a fuel flow passage to said engine downstream fromsaid fuel control.
 3. An engine autotrim system as in claim 1 furthercomprising a limit switch connected with each of said fuel trimmingmeans, said limit switch being closed and producing a limit signal onlywhen the respective fuel trimming means is at the limit of its fueladjustment range whereby the minimum fuel flow passes therethrough, andmeans responsive to the absence of a limit signal from any of said limitswitches to actuate the fuel trimming means for all of said engines in adirection to reduce the flow of fuel therethrough.
 4. An engine autotrimsystem as in claim 3 further comprising means for disconnecting thepressure ratio error signal from all of said fuel trimming means whenall of said limit switches are open.